The present invention relates to apparatus and methods for collecting and transferring solids, including grit, that have been separated out of raw water or wastewater influent in a generally quiescent settling process.
As used herein, “grit” refers to high-density, inorganic, settleable particles, found in an influent stream (i.e., after the raw water or the raw sewage has been screened but typically before subsequent processing). Grit causes wear to downstream mechanical treatment equipment and, if it accumulates, loss of performance.
One type of apparatus used for separating grit from influent is referred to as a head cell. The hydraulic separation action in a head cell occurs through controlling the influent to flow at predetermined speeds and along a predetermined course, and does not require the use of chemicals. The influent enters at the periphery or rim of a funnel-like conical surface from a direction tangential to the rim, and then flows over and around the downwardly sloping conical surface, at least partially circling a centrally located opening. The flow conditions are determined such that a dynamic boundary layer is developed at the conical surface.
As the influent flows around the downwardly sloping conical surface, the grit is separated out onto the conical surface. At the same time, the remaining liquid, i.e., the effluent (which is relatively grit-free), is guided to flow out of the head cell through openings located at the outer periphery of the conical surface. In general, this effluent is channeled for further treatment downstream.
The separated grit moves downwardly along the sloping conical surface and through the central opening for collection at a point beneath the opening.
A head cell may have several individual conical surfaces or “trays” that are vertically aligned with each other such that grit draining through the central opening in an upper tray also passes through similar central openings in all lower trays. The separated grit is collected at a point beneath the lowest tray.
According to another conventional approach, a vortex separator is used both to (1) separate the grit from the influent (as an alternative to the head cell described above) and (2) collect and transfer the grit. In a conventional vortex separator 110, as shown in FIG. 5, influent is channeled through an inlet trough 140 into and around a settling chamber 111, grit is separated from the influent in the settling chamber 111 under the action of boundary layer flow established by a rotating blade 117, and the resulting effluent is channeled out of the separator 110 through an outlet trough 153.
The influent travels downwardly, and then spirals upwardly over the course of approximately 450 degrees before exiting the settling chamber 111. The boundary layer flow causes the grit to travel from the periphery of the settling chamber 111, where separation begins, towards the center of the chamber 111, and downwardly through a central opening 115 in the settling chamber 111, to a grit storage chamber 112 positioned below the chamber 111.
After grit begins accumulating in the grit storage chamber 112, it must be transferred out of the separator 110. In the conventional separator 110, a pump (not shown) lifts the grit particles up from the bottom of the chamber 112 and propels them upward through a pipe 147 and out of the separator 110. A similar conventional approach could be used to transfer accumulated grit from a grit storage chamber 60 of the conventional head cell 10 as shown in FIG. 2D.
This conventional approach, however, is subject to failure. For example, if too much grit accumulates in the grit storage chamber, e.g., in the case of a heavy concentration of grit or a power failure that prevents operation of the pump, the pipe 147 may become clogged. Occasionally, objects in the influent other than grit (e.g., rags) also may foul the pipe 147.
In these situations, the separating apparatus must be shut down and partially dismantled to allow access to the grit storage chamber and pump for manual defouling. The apparatus cannot be defouled simply by restoring operation to the pump or operating it in a different mode. Manual defouling is costly and very inconvenient. Some installations even provide for backup vortex separators in anticipation of such fouling problems, which represents an additional cost in providing redundant equipment.
Therefore, it would be desirable to provide an apparatus for collecting and transferring grit out of the grit storage chamber that is efficient, highly reliable and not subject to regular fouling. It would also be desirable to provide an apparatus that is self-clearing.